The invention generally relates to conductive polymeric and elastomeric materials for use in a wide variety of applications, including without limitation, conductive adhesives, conductive gaskets and conductive films.
The design of an electrically conductive pressure sensitive adhesive (PSA), for example, has long presented challenges at least because adhesive strength and flexibility generally decrease with increased electrical conductivity. The materials that are typically used (added) to provide good electrical conductivity are generally less flexible and inhibit adhesion. A conventional way to prepare a conductive coating is to fill a polymeric material with conductive particles, e.g., graphite, silver, copper, etc., then coat, dry and cure the polymeric binder. In these cases the conductive particles are in such a concentration that there is a conductive network formed when the particles are each in physical contact with at least one other neighboring particle. In this way, a conductive path is provided through the composite.
For pressure sensitive adhesives, however, if the particle concentration is high enough to form a network in which particle-to-particle contact is maintained then there is little chance that the polymer (e.g., elastomer) system of the PSA component is present in high enough concentrations to flow out to make surface-to-surface contact between the substrates and an electrode, i.e., act as an adhesive. Conversely, if the PSA component is in sufficient concentration to make sufficient surface contact to the substrate, the PSA would have to interrupt adjacent conductive particles such that particle-to-particle contact is disrupted.
Another type of electrically conductive PSA includes conductive spherical particles with diameters equal to or greater than the thickness of the PSA. In this fashion the signal or current may be carried along the surface of the particles, thus providing current flow anisotropically in the z dimension of the adhesive. The continuity of the adhesive however, may be compromised.
Salts, such as sodium or potassium chloride, readily dissolve when in an aqueous medium, and their ions dissociate (separate into positive and negative ions). The dissociated ions may then convey an electrical current or signal. For this reason, salts have long been added to water, which then may be added to polymeric and elastomeric materials, to provide good electrical conductivity. For example, U.S. Pat. No. 6,121,508 discloses a pressure sensitive adhesive hydrogel for use in a biomedical electrode. The gel material is disclosed to include at least water, potassium chloride and polyethylene glycol, and is disclosed to be electrically conductive. U.S. Pat. No. 5,800,685 also discloses an electrically conductive adhesive hydrogel that includes water, salt, an initiator or catalyst and a cross linking agent. The use of such hydrogels however, also generally requires the use of a conductive surface at one side of the hydrogel (away from the patient) that is capable of receiving the ionically conductive charge, such as silver/silver chloride, which is relatively expensive.
While these hydrogel/adhesives can have good electrically conductive properties, they often have only fair adhesion properties. Another downside is that the electrical conductivity changes with changing water content, such as changes caused by evaporation, requiring that the hydrogels be maintained in a sealed environment prior to use, and then used for a limited period of time only due to evaporation.
U.S. Pat. No. 7,651,638 discloses a water insensitive alternating current responsive composite that includes a polymeric material and a polar material (such as an organo salt) that is substantially dispersed within the polymeric material. The polymeric material and the polar material are chosen such that they each exhibit a mutual attraction that is substantially the same as the attraction to itself. Because of this, the polar material neither clumps together nor blooms to a surface of the polymeric material, but remains suspended within the polymeric material. This is in contrast to the use of salts in other applications wherein the salt is intended to bloom to the surface (to provide a conductive layer along a surface, e.g., for static discharge).
U.S. Pat. No. 5,082,595 discloses an electrically conductive pressure sensitive adhesive that includes carbon particles, and the conductive adhesive is disclosed to be prepared by incorporating black filler (carbon) into the pressure sensitive adhesive in such a manner as to impart electrical conductivity, yet have a concentration low enough to avoid adversely affecting the physical properties (such as tack) of the adhesive. In particular, this patent states that slurry of the carbon black in an organic solvent is formed under mild agitation or stirring in the absence of high shear, so that carbon structures are thereby formed. The mixture may then be introduced into an adhesive. Such a composite, however, may not provide sufficient adhesiveness and conductivity in certain applications.
Such composites may also include areas with relatively greater or lesser concentrations of structures of carbon black. Certain conductive polymeric and elastomeric materials that include conductive particles in concentration within the polymeric or elastomeric material, may therefore exhibit inconsistent electrical properties over the surface of the material.
There remains a need therefore, for a composite for use as a conductive polymeric material that provides electrical conductivity without compromising the desired properties of the polymeric material, and further, there is a need for conductive polymeric materials that provide consistent electrical characteristics.